This invention relates to a method of specifying a three-dimensional curve and, more particularly, to a method of specifying a three-dimensional curve that is ideal for the creation of an NC tape required for the numerically controlled machining of a three-dimensional metal mold or the like.
A curved surface of a three-dimensional metal mold or the like on a design drawing is generally expressed by a plurality of section curves, but no profile data is shown for the shape of the area lying between a certain section curve and the next adjacent section curve. In numerically controlled machining it is essential that machining be carried out so as to smoothly connect these two section curves despite the fact that the profile between them is not given. In other words, this means that machining must be performed by generating the curved surface between the two section curves from such data as that indicative of the section curves, recording on an NC tape the data concerning the generated curved surface, and carrying out machining in accordance with commands from the NC tape. To this end, the applicant has proposed in U.S. Ser. No. 266,284 now U.S. Pat. Nos. 4,491,906 and 4,569,014 (see Japanese Patent Application No. 55-78247), and has put into practice, a method of creating curved surfaces, comprising generating a plurality of intermediate sections and finding a section curve (intermediate section curve) on a curved body, based on the intermediate sections, in accordance with predetermined rules, from section data specifying given sections of a three-dimensional curved body and from data specifying section curves in said sections, and generating the curved surface of the three-dimensional body based on the plurality of generated intermediate section curves. This prior-art method will be described with reference to FIG. 1. The method includes giving section curves 11a, 12a in two sections 11, 12 and a reference curve 21a in a reference section 21, moving the first section curve 11a of the two given section curves 11a, 12a along the reference curve 21a while the curve is being transformed so as to be superposed on the second section curve 12a, and creating a curved surface, which is formed owing to said movement, as an assemblage of a plurality of intermediate section curves. In generating the intermediate section curves, the first and second section curves 11a, 12a are brought into uniform correspondence with each other in their entirety. Specifically, when each of the section curves is partitioned into M segments, i-th (i=1, 2, . . . ) partition points P.sub.i, Q.sub.i of the respective curves are made to correspond to each other and each of the intermediate sections curves is generated by using the corresponding relation.
However, since the prior-art method is adapted to generate a curved surface by giving the reference curve 21a in the reference section (a plane), curved surface generation cannot be performed in a case where a reference curve 31a is a three-dimensional curve, as shown in FIG. 2. In FIG. 2, numerals 11, 12 denote sections, 11a, 12a section curves, 41 an intermediate section, and 41a an intermediate section curve.